Photocatalytic Aerobic Dehydrogenation of N-Heterocycles with Ir(III) Photosensitizers Bearing the 2(2′-Pyridyl)benzimidazole Scaffold

Photoredox catalysis constitutes a very powerful tool in organic synthesis, due to its versatility, efficiency, and the mild conditions required by photoinduced transformations. In this paper, we present an efficient and selective photocatalytic procedure for the aerobic oxidative dehydrogenation of partially saturated N-heterocycles to afford the respective N-heteroarenes (indoles, quinolines, acridines, and quinoxalines). The protocol involves the use of new Ir(III) biscyclometalated photocatalysts of the general formula [Ir(C^N)2(N^N′)]Cl, where the C^N ligand is 2-(2,4-difluorophenyl)pyridinate, and N^N′ are different ligands based on the 2-(2′-pyridyl)benzimidazole scaffold. In-depth electrochemical and photophysical studies as well as DFT calculations have allowed us to establish structure–activity relationships, which provide insights for the rational design of efficient metal-based dyes in photocatalytic oxidation reactions. In addition, we have formulated a dual mechanism, mediated by the radical anion superoxide, for the above-mentioned transformations.


■ INTRODUCTION
N-heterocycles are pivotal scaffolds in the pharmaceutical industry due to their biological activity and medicinal applications. 1 In particular, indoles, 2 quinolines, 3,4 acridines, 5,6 and quinoxalines 7 display anticancer, antibiotic, antibacterial, antifungal, and anti-inflammatory properties. Moreover, the redox couples formed by 1,2,3,4-tetrahydroquinolines (THQ) and the corresponding quinolines have been proposed as potential hydrogen-storage material systems for fuel cell applications, since the catalytic hydrogenation of quinolines takes place under mild reaction conditions and can be reverted through catalytic dehydrogenation protocols. 8 Traditional procedures for preparing N-containing aromatic molecules from partially saturated N-heterocycles involve harsh reaction conditions (high temperatures), the use of stoichiometric toxic or corrosive oxidants (2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ), sulfur, or metal oxides), as well as the generation of undesirable waste. 9 More recently, several groups have described methodologies to prepare different aromatic N-heterocycles (N-heteroarenes) from partially saturated precursors through either catalytic dehydrogenation 10 or catalytic acceptorless dehydrogenation. 11,12 Nevertheless, both strategies require high temperatures and/or harsh reaction conditions and, in some cases, harmful solvents and high catalyst loadings.
The synthesis of N-heteroarenes can also be accomplished through photocatalytic approaches such as the acceptorless dehydrogenation (ADH) of THQs, indolines and similar heterocycles. Different photocatalytic systems have been successfully used to prove this methodology, namely, combinations of a Ru-photocatalyst (PC) and a Co catalyst, 8,13 or an acridinium PC and a Pd metal catalyst, 14 and also heterogeneous PCs, that is, hexagonal boron carbon nitride nanosheets 15 or Rhphotodeposited TiO 2 nanoparticles. 16 This transformation produces molecular hydrogen as the only byproduct, but it must be managed through expensive procedures when operating at high scale.
Alternatively, it is possible to access N-heteroarenes through oxidative dehydrogenation (ODH) of partially saturated precursors under aerobic photocatalytic conditions, which implies the use of O 2 as the hydrogen acceptor (green oxidant), visible light, and a photosensitizer. In particular, the synthesis of a variety of N-heteroarenes (quinolines, quinoxalines, quinazo-lines, acridines, and indoles) has been performed using this type of strategy in the presence of different photocatalytic systems: [Ru(bpy) 3 ]Cl 2 , 17 Rose Bengal, 9 TiO 2 grafted with Ni(II) ions in the presence of 4-amino-TEMPO, 18 and a cobalt-phthalocyanine photoredox catalyst in a biphasic medium. 19 Nevertheless, there is still scope to explore new photosensitizers with the goal of increasing product yields, reducing reaction times and employing solvents with low boiling points. What is more, additional studies should be done for a better understanding of the reaction mechanism entailed in this type of transformations.
In a previous work, we have designed a family of new Ir(III) biscyclometalated complexes with β-carbolines as efficient photocatalysts for the one-pot oxidative thiocyanation of indolines, which produces the respective 3-thiocyanate indoles. 20 We have also reported on a protocol to prepare αamino nitriles through the Ru-photosensitized oxidative cyanation of amines. 21 In this work, we present the synthesis of new Ir(III) biscyclometalated complexes of general formula [Ir-(C^N) 2 (N^N′)]Cl, where C^N = 2-(2,4-difluorophenyl)pyridinate (dfppy) and N^N′ stands for different N,N-donor ligands containing the 2-(2′-pyridyl)benzimidazole scaffold. The ligand dfppy was chosen to obtain enhanced photoluminescent quantum yields and excited-state lifetimes, since this behavior is usually expected from the presence of electronwithdrawing groups, such as the −F atoms on the C^N ligands in this type of complexes. 22, 23 2-(2′-pyridyl)benzimidazole was selected as the scaffold for the N^N′ ligands due to both its commercial availability and the presence of the imidazole N−H, which allows easy functionalization with a variety of alkyl groups. This, in turn, allows to explore the impact of different functional groups on the photophysical and photocatalytic properties of the resulting complexes (see below). In addition, we describe the evaluation of these complexes as photosensitizers in dehydrogenation processes. Furthermore, relationships between the photosensitizing abilities of these complexes and their electrochemical and photophysical properties are established. In particular, the effect of using dfppy as the C^N ligand and the influence of the different functional groups of the N^N′ ligands on the photocatalytic performance of our dyes are emphasized.

■ RESULTS AND DISCUSSION
Synthesis of Ligands and Iridium(III) Complexes. We have synthesized a family of Ir(III) biscyclometalated compounds of general formula rac-[Ir(C^N) 2 (N^N′)]Cl with the aim of developing new efficient photocatalysts. In this series of compounds, we have furnished the iridium center with two units of the anionic C^N donor 2-(2,4-difluorophenyl)pyridinate (dfppy) and five different N^N′ ligands based on the 2-(2′-pyridyl)benzimidazole scaffold (Hpybim = L1). The ligand 2-(2′-pyridyl)benzimidazole (L1) is commercially available, and its N-functionalized derivatives (L2−L5) were prepared by reacting L1 with MeI, for L2, or the appropriate alkyl bromide (R-Br), for L3−L5, at room temperature in the presence of K 2 CO 3 , using DMF as solvent (see Figure 1). 24−26 The incorporation of diverse alkyl groups into the N^N′ ligand aimed to reduce intermolecular interactions and to assess different effects on the photophysical and photocatalytic properties of the resulting Ir derivatives. Thus, the methyl and benzyl groups (L2, L3, and L4) were chosen to protect the respective complexes from either self-quenching or N−H reactivity. The naphthalenylmethyl group (L5) was used to evaluate the potential beneficial effect of a π-extended system on the absorption profile of its Ir derivative.
The  (Figure 1). The products were isolated in the form of bright yellow solids, as chloride salts of racemic mixtures corresponding to the Δ and Λ cationic complexes (helical chirality).
The synthesis of the PF 6 − salts of complexes [Ir1] + and [Ir2] + has been previously described, but to the best of our knowledge, their photocatalytic activity has not been studied so far. 27,28 Characterization of the Ir(III) Complexes. The iridium derivatives were unequivocally characterized by multinuclear NMR, mass spectrometry, elemental analysis, and IR spectroscopy. In  The 19 F NMR spectra of all the derivatives feature two quartets in the range between −106.5 and −107 ppm (F 11 and F 11' ) and two triplets at about −109 ppm (F 9 and F 9' ), for the two nonequivalent dfppy (see atom numbering in the Supporting Information (SI)).
The HR-MS (ESI+) spectra of the Ir(III) complexes present peaks where the m/z values and the isotopic patterns match unambiguously with those calculated for the monocationic species of general formula [Ir(dfppy) 2 (N^N′)] + (N^N′ = L1−   Figure 2. Selected bond distances and angles along with standard deviations are collected in Table 1, and relevant crystallographic parameters are included in Table S1. The molecular structures of these complexes display a pseudo-octahedral geometry with the well-known trans-N,N and cis-C,C arrangement for the C^N ligands ( Figure 2). In all the derivatives, the Ir−N bond distances for the C^N ligands (1.963(12)−2.074(12) Å) are shorter than for the N^N′ ligands (2.119(6)−2.181(7) Å) as a consequence of the strong trans influence exerted by the coordinated phenyl rings. 30−33 Besides, the Ir−N bim length is shorter than the Ir−N py in the N^N′ ligand of every complex, likely due to the bigger π-electron density on the benzimidazole (bim) fragment relative to the pyridine (py) ring and therefore the higher π-donor ability of bim versus py. The Ir−C bond distances are standard (1.995(13)−2.020(6) Å). 34,35 The torsion angles for the C^N and the N^N′ ligands, C−C−C−N (−0.02 to −6.22°) and N−C−C−N (1.57 to −18.99°), are small, which in practice underlines the coplanarity of the metallacycles.
Photostability Experiments. In order to verify the photostability in solution of the new Ir(III)-complexes and the standard photocatalysts [Ir(ppy) 2 Tables S2a  and S2b.
The HOMOs calculated for [1] + , [2] + , and the new derivatives are formed by a combination of Ir orbitals (d π ) and C^N orbitals (π of ppy − or dfppy − ) as described elsewhere for this type of complexes. 20,36,37 Hence, the HOMOs are located on the Ir metal center and the phenyl rings of the C^N ligands, although they exhibit a π-antibonding nature at the Ir−C phenyl interfaces. On the contrary, the LUMOs are distributed mainly over the N^N′ ligands (bpy or 2-(2′-pyridyl)benzimidazole    Figure 3 and Figure S23a), but are noticeably lower than the energy obtained for the HOMO of [1] + (−5.65 eV). This effect is ascribed to the electron-withdrawing ability of the −F atoms in dfppy, which leads to a remarkable stabilization of the HOMO in 28,40 The energies calculated for the LUMOs of [Ir1] + −[Ir5] + and [2] + are also very similar (from −2.45 to −2.49 eV, Figure 3 and Figure S23) and slightly lower than that estimated for the LUMO of These bands are attributed to mixed spin-allowed 1 MLCT and 1 LLCT transitions. The weak absorption tails entering in the visible region come from spin-forbidden 3 MLCT and 3 LC transitions. 41−43 In general, the absorption bands of [Ir5]Cl are more intense and are more extended in the range between 420 and 500 nm, and hence overlap better with the emission band of the light source used in photocatalytic assays ( Figure 5a). This is likely due to the higher π-conjugation of the naphthyl group.
Emission Spectra. The emission spectra of complexes [Ir1]Cl−[Ir5]Cl were recorded in solutions of dry and deoxygenated acetonitrile (10 −5 M) at 25°C under excitation at 405 nm (see Figure 5b). All the spectra are alike, featuring a broad unstructured emission band, typical of high chargetransfer character. 40 These bands have an absolute maximum between 522 and 546 nm for [Ir1]Cl−[Ir5]Cl (Table 2), which resembles the value reported for [2]PF 6 (λ em = 534 nm).   Nevertheless, the emission of all these complexes is blue-shifted relative to that of the archetypal photosensitizer [1]PF 6 (602 nm), as anticipated by DFT calculations.
The photoluminescence quantum yields (PLQY, Φ PL ) were also determined in deoxygenated acetonitrile solutions ( Cl are mainly due to the intramolecular rotation of the N-alkyl groups in solution, which favors the dissipation of energy by nonradiative channels for these complexes. 44,45 In addition, the very low PLQY of [Ir5]Cl could be the result of an extra factor, that is, the thermal population of a ligand-centered ( 3 LC, π L5 → π* L5 ) excited state, (T 2 , 2.70 eV) close in energy to the emissive lowest excited state (T 1 , 2.65 eV) (Table S3). This feature provides a nonradiative decay pathway to [Ir5]Cl, since the nonparticipation of the metal center in T 2 hampers the intersystem crossing process, and hence a low PLQY is observed. 46 Therefore, we conclude that the functional group on the N^N′ ligand exerts an important influence in the efficiency of the emission process.
The excited-state lifetimes (τ) are excellent for the substituted derivatives [Ir2]Cl−[Ir5]Cl, between 1012 and 2066 ns and much longer than that for [1]PF 6 , whereas for the nonfunctionalized compound, [Ir1]Cl, τ is much shorter, 59 ns (Table 2). Hence, the functionalization of the imidazolyl nitrogen has also an important effect on the lifetimes of the excited states. In particular, we speculate that the presence of the N−H group in [Ir1]Cl could accelerate the radiative deactivation of the excited state relative to its functionalized counterparts [Ir2]Cl−[Ir5]Cl. The rationale for this could be that the ground state (S 0 ) of [Ir1] + is stabilized in acetonitrile solution through N−HCl − or N−HNC−Me hydrogenbonding interactions. By contrast, in the excited state, which exhibits partial 3 MLCT nature, the charge transfer from the metal center to the π* orbital of the N^N′ ligand decreases the polarization of the N−H bond and therefore the strength of the interaction with either the Cl − counterion or the solvent molecules, shortening the lifetime of the triplet excited state (T 1 ). In [Ir2]Cl−[Ir5]Cl, the presence of bulky apolar alkyl groups impedes hydrogen-bonding interactions and therefore avoids the differential stabilization of S 0 relative to T 1 . This would explain the longer lifetimes observed for the excited states The radiative and nonradiative deactivation rate constants, k r and k nr , were calculated from Φ PL and τ and are summarized in Table 2. It is worth noting that [Ir1]Cl has a k r < k nr , while The cyclic voltammograms (CV) of these compounds are presented in Figure 6. The anodic region of every CV shows two peaks: (a) an irreversible peak between +0.56 and +0.63 V (Table 3 and Figure 6) attributed to the oxidation of the chloride counteranion (2 Cl − → Cl 2 + 2 e − ) and (b) a reversible oneelectron oxidation peak in the range +1.19 to +1.22 V, ascribed to an oxidation process affecting the Ir(III) center along with the difluorophenyl rings of C^N ligands, 28 as disclosed by the topology of the respective HOMO.
In the cathodic region, [Ir1]Cl exhibits two irreversible peaks Cl display one pseudoreversible one-electron peak and one irreversible one-electron peak in the ranges from −1.67 to −1.76 V (E 1/2 red1 ) and from −2.21 to −2.27 V (E 1/2 red2 ), respectively. These waves are attributed to stepwise reductions centered in the respective N^N′ ligands, as suggested by the topology of the calculated LUMO for these compounds. Interestingly, the pseudoreversible nature of E  6 in agreement with the trends predicted theoretically for the HOMO−LUMO band gaps. Paradoxically, the excited states of this type of Ir(III) derivatives exhibit a versatile and outstanding redox behavior. 50−52 Indeed, our dyes show a high excited-state redox power as oxidants, E 1/2 (Ir III */ Ir II ) ranges from +0.54 to +0.61 V, and also as reductants, E 1/2 (Ir IV /Ir III *) ranges from −1.16 to −1.05 V (Figure 7 and Table S4), and they are meaningfully better excited-state oxidants than the standard photosensitizer [1](PF 6 ) (E 1/2 (Ir III */Ir II ) = +0.28 V). 53 These facts underscore their potential as photocatalysts in single electron transfer (SET) processes.
Photocatalytic Activity in the Oxidation of Heterocycles. The new iridium complexes were tested as photo-   Table 5). We tentatively explain the poor yield obtained with     Figure 6 and discussed in the Mechanism section. By contrast, it is worth remarking that the alkylated derivatives, [Ir3]Cl−[Ir5]Cl, provide better yields than the standard PSs, [1]Cl and [2]Cl, as a result of a balanced combination of favorable photophysical and electrochemical properties.
Next, we performed a collection of control experiments to verify the photocatalytic essence of this transformation and the role of O 2 . Indeed, we realized that in the absence of light, PC, or O 2 (N 2 atmosphere), the reaction did not proceed or was dramatically impeded, and thus we concluded that this transformation is light-driven in the presence of a PC and that oxygen is involved in the oxidation ( Table 6). It is worth mentioning that the detection of a small percentage of 2a under a N 2 atmosphere (entry 4, Table 6) could be due to the presence of O 2 traces in the solvent (incomplete deoxygenation). Moreover, we carried out additional control experiments in the presence of DABCO ( 1 O 2 quencher), 57 TEMPO (radical scavenger), 58

and 1,4-benzoquinone (BQ, O 2
•− scavenger) 56,59 to elucidate the actual oxidant. The presence of DABCO decreases the yield slightly (84%), while TEMPO and BQ cause a dramatic and significant drop of the yield, respectively. These results suggest that superoxide has a major contribution in this reaction, while singlet oxygen plays just a minor role (Table 6).
Then, we tested the substrate scope using the optimized conditions on a variety of indolines bearing different functional groups ( Table 7). Most of the desired indoles were obtained in high yields and with excellent selectivities. However, the oxidation of 1-acetyl-5-bromoindoline (1f) was ineffective. This failure is likely due to the electron-withdrawing and steric effects attributed to the N-acetyl group, which inhibit the oxidation step. 8,16,18 Indeed, according to our general mechanistic proposal, we presume that the reductive quenching of the triplet excited state of the PC, 3 [Ir III ]*, in the presence of 1f would give rise to an unstable radical cation intermediate due to the remarkable electron-withdrawing effect attributed to the formyl substituent on the N atom. Moreover, the oxidative dehydrogenation of 5-nitroindoline (1c) and 6-nitroindoline (1g) were also precluded (0 and 20% of respective indoles), which is likely related to the strong electron-withdrawing ability of the −NO 2 group. 18 Indeed, it is well-known that electronpoor nitro-aromatic substrates can undergo a photoinduced electron donation from the triplet excited state of different photosensitizers, which competes with the photoinduced reductive quenching proposed as one of the steps in the mechanism of this reaction. 60,61 To validate the applicability of this protocol, we decided to scale the reaction up to 1 g of indoline (1a) in the presence of [Ir3]Cl (0.3 mol %). Thus, it was possible to obtain 2a in 95% yield by increasing the reaction time from 24 to 75 h. (see SI and 1 H and 13 C NMR of isolated products and characterization in Figures S19−S32).
Again, acetonitrile provided the best yield for quinoline, 4a, (20%) and was chosen as the solvent for additional experiments. Partial dehydrogenation products such as 3,4-dihydroquinoline were not detected, making this protocol selective. 62 [2]Cl. We theorize that the good performance of [Ir5]Cl, despite its low Φ PL , could be ascribed to its better absorptivity in the visible range.
The usual control experiments were done to gain insight into the mechanism of this transformation. In particular, we observed no conversion without light or PC as well as a drastic decrease in the yield in the absence of O 2 (4% of 4a, under a N 2 atmosphere) ( Table 10). The use of the ROS scavengers DABCO, TEMPO, and BQ revealed similar behaviors to those established for the photooxidation of indoline, that is, a slight drop in the yield of 4a in the presence of DABCO (87%), but a severe inhibition of the transformation in the presence of TEMPO (7%) and BQ (17%) relative to the standard conditions (entries 1 and 5−7 in Table  10). In conclusion, we propose that both singlet oxygen and superoxide take part in the dehydrogenation reaction of 1,2,3,4tetrahydroquinoline, although the main role would correspond to the radical anion superoxide (O 2 •− ). To gain additional insight into the reaction mechanism, we performed emission quenching Stern−Volmer experiments. Thus, we could determine that phosphorescence of [Ir4]Cl was strongly quenched in the presence of increasing concentrations of 3a under nitrogen, and consequently we proved that reductive quenching can be rationally proposed as the first step in the mechanism of this transformation. In other words, we concluded  Figure S47). Hence, oxidative quenching   To complete this study, we extended the above-mentioned protocol to a selection of tetrahydroquinolines and analogues, such as 1,2,3,4-tetrahydroisoquinoline, 9,10-dihydroacridine and several 1,2,3,4-tetrahydroquinoxalines (Table 11).
In general, we obtained high yields and excellent selectivity for most of the expected products (4b, 4c, and 4e−4h). In a previous photocatalytic protocol, Bahnemann et al. obtained a mixture between the partially dehydrogenated product 4b and the fully dehydrogenated product, when using the tetrahydroisoquinoline 3b. 18 However, the yields for the quinoxalines, 4i−4k, and 6-methyl-quinoline, 4f, were only moderate, in the range between 52 and 62%. On the other hand, 2,3dihydrobenzofuran-5-carboxaldehyde (3d) was not oxidized to its dehydrogenated derivative. It is noteworthy that the oxidation of 7-nitro-1,2,3,4-tetrahydroquinoline (3c) was achieved albeit with a low yield, since, as aforementioned, the nitro substituent usually behaves as a quencher for the excited state of PCs. Moreover, the yield for 3c could be improved by prolonging the reaction time and increasing the catalyst loading (>99% yield, with 5 mol % PC, 48 h).
After this, we successfully scaled our methodology up to 1 g of 3a in the presence of [Ir4]Cl (0.7 mol %) to obtain 4a with a yield of 88%, albeit it was necessary to extend the reaction time from 24 to 75 h (see 1 H and 13 C NMR spectra of isolated products in Figures S42 and S43).
Mechanism. Based on the experimental results summarized in Table 12 along with the bibliographic background, 17 we propose a dual mechanism for the aerobic photooxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline based on both a reductive quenching cycle (pathway A) and simultaneously on an oxidative quenching cycle (pathway B). In both cases, the reaction is mediated by the radical anion superoxide (O 2 •− ), and we postulate that both mechanisms could operate concurrently (Figure 9).
Pathway A. First, the model Ir(III) photosensitizer, [Ir3]Cl, is promoted to the singlet excited state under irradiation with blue light and then is converted to the respective triplet excited state through intersystem crossing. This species, 3 [Ir III ]*, exhibits a high oxidation ability and therefore is capable of generating the radical cation intermediate species A (THQ + ), through a SET, which entails a reductive quenching of the excited state. The redox potential of the couple THQ/THQ + was determined by CV, E(THQ/THQ + ) = −0.16 V vs Fc + /Fc ( Figure S24) and compared to E(Ir III */Ir II ) = +0. 59  •− and species A react to give B and C, as explained above for pathway A. A similar mechanism could operate for the photocatalytic aerobic dehydrogenation of indolines, etc.

■ CONCLUSIONS
In conclusion, we have designed and prepared a new family of Ir(III) photosensitizers of the general formula [Ir-(C^N) 2 (N^N′)]Cl, where C^N = 2-(2,4-difluorophenyl)pyridinate and N^N′ = 2-(2′-pyridyl)benzimidazole (L1) or its N-alkylated derivatives L2-L5. We have ascertained that these complexes are notably stable under irradiation with blue light for a period of 24 h. Moreover, we have demonstrated that they absorb weakly in the visible light region and can be excited with blue light. Indeed, all of them are emissive in the range between 522 and 546 nm (λ ex = 405 nm). In particular, the Nfunctionalized derivatives, [Ir2]Cl−[Ir5]Cl, exhibit moderate or high PLQYs (9−63%) and very long excited-state lifetimes (1012−2066 ns). On the contrary, the nonalkylated compound, [Ir1]Cl, features an excellent PLQY (78%) but a very short excited-state lifetime (59 ns). This divergent behavior suggests that the N−H group speeds up the radiative deactivation of the excited state for [Ir1]Cl, by stabilization of the ground state through hydrogen bonds with counterion/solvent molecules, whereas the replacement of the N−H with apolar N-alkyl groups prevents this effect on the ground state and lengthens the lifetime of the respective excited states in acetonitrile. Regarding their electrochemical properties, all the Ir complexes display a similar redox behavior, with electrochemical band-gaps higher than that determined for the standard photosensitizer [Ir-(ppy) 2 (bpy)]PF 6 , [1]PF 6 . This is due to the strong stabilization of the HOMO, associated with the presence of electrowithdrawing −F atoms in the C^N ligands of our PS, as revealed by theoretical calculations. Nevertheless, [Ir1]Cl features an irreversible E 1/2 red1 in contrast to the reversible E 1/2 red1 of its derivatives. Upon excitation with blue light, these compounds exhibit highly efficient and selective photocatalytic activities in the preparation of a wide variety of aromatic N-heterocyclic   66 Furthermore, we propose that these Ir-photosensitized transformations occur through a dual mechanism based on both a reductive quenching cycle (pathway A) and an oxidative quenching cycle (pathway B) which operate simultaneously and are mediated by the radical anion superoxide (O 2 •− ). To summarize, we have shown that the easy N-alkylation of 2-(2′-pyridyl)benzimidazole affords ligands suitable for the assembly of Ir(III) photosensitizers, [Ir2]Cl−[Ir5]Cl, which feature ideal properties to be used in photoredox catalysis. Indeed, these PSs exhibit highly efficient and selective photocatalytic activities in the preparation of a wide variety of N-heterocyclic products through oxidative dehydrogenation of partially saturated substrates. The above-mentioned results provide insights and tools for the rational design of efficient photocatalysts.
■ EXPERIMENTAL SECTION General Information and Procedures. All synthetic manipulations were carried out under an atmosphere of dry, oxygen-free nitrogen using standard Schlenk techniques. The solvents were dried and distilled under nitrogen atmosphere before use. Elemental analyses were performed with a Thermo Fisher Scientific EA Flash 2000 elemental microanalyzer. IR spectra were recorded on a Jasco FT/IR-4200 spectrophotometer (4000−400 cm −1 range) with single reflection ATR measuring attachment. UV−vis absorption was measured in an Evolution 300 UV−vis double beam spectrophotometer (Thermo Scientific). Fluorescence steady-state and lifetime measurements were performed in a FLS980 (Edinburg Instruments) fluorimeter with xenon arc lamp 450W and TCSPC laser, respectively. Quantum yield was determined by using in a FLS980 (Edinburg Instruments) with xenon arc lamp 450W and Red PMT Sphere as detector. HR-ESI(+) mass spectra (position of the peaks in Da) were recorded with an Agilent LC-MS system (1260 Infinity LC/6545 Q-TOF MS spectrometer) using DCM/DMSO (4:1) as the sample solvent and (0.1%) aqueous HCOOH/MeOH as the mobile phase. The experimental m/z values are expressed in Da compared with the m/z values for monoisotopic fragments. NMR samples were prepared by dissolving the suitable amount of compound in 0.5 mL of the respective deuterated solvent, and the spectra were recorded at 298 K on a Varian Unity Inova-400 (399.94 MHz for 1 H; 376.29 MHz for 19 F; 100.6 MHz for 13 C). Typically, 1 H NMR spectra were acquired with 32 scans into 32,000 data points over a spectral width of 16 ppm. 1 H and 13 C{ 1 H} chemical shifts were internally referenced to TMS via the residual 1 H and 13 C signals of DMSO-d 6 (δ = 2.50 ppm and δ = 39.52 ppm), CD 3 CN (δ = 1.94 ppm and δ = 118.69 (−CN) and 1.39 (−CD 3 ) ppm) and CDCl 3 (δ = 7.26 ppm and δ = 77.16 ppm), according to the values reported by Fulmer et al. 1 Chemical shift values (δ) are reported in ppm and coupling constants (J) in hertz. The splitting of proton resonances in the reported 1 H NMR data is defined as s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, bs = broad singlet. 2D NMR spectra such as 1 H− 1 H gCOSY, 1 H− 1 H NOESY, 1 H− 13 C gHSQC, and 1 H− 13 C gHMBC were recorded using standard pulse sequences. The probe temperature (±1 K) was controlled by a standard unit calibrated with methanol as a reference. All NMR data processing was carried out using MestReNova version 10.0.2.
Starting Materials. IrCl 3 ·xH 2 O was purchased from Johnson Matthey and used as received. The starting dimer ([Ir(μ-Cl)-(dfppy) 2 ] 2 ) (dfppy = 2-(2,4-difluorophenyl) pyridinate) was prepared according to the reported procedure. 2 The reagents 2-(2,4difluorophenyl)pyridine), iodomethane, and benzyl bromide were purchased from Sigma-Aldrich, 2-(2-pyridyl)benzimidazole and 4iodobenzyl bromide were purchased from Acros Organics-Fisher Scientific, and 2-(bromomethyl)naphthalene was purchased from Alfa Aesar. All of them were used without further purification. Deuterated solvents (DMSO-d 6 , CDCl 3 , CD 3 CN) were obtained from Eurisotop. Conventional solvents such as diethyl ether (Fisher Scientific), acetone (Fisher Scientific), and 2-ethoxyethanol (Across Organics) were  Tables 6 and 10 low photocatalytic activity obtained for [Ir1]Cl, due to irreversible reductive quenching screening of photocatalysts (Tables 5 and 9) and redox potentials (Table 3) low dehydrogenation for 1c, 1g, and 3c, due to the presence of − NO 2 groups which induce oxidative quenching on PS and inhibit the photocatalytic quenching steps substrate scope experiments (Tables 7 and 11) suitable redox potentials for sustaining both a reductive quenching cycle and an oxidative quenching cycle see text in this section and Tables 3 and S4  detection Table S1. Single crystals of compounds were coated in high-vacuum grease, mounted on a glass fiber, and transferred to a Bruker SMART APEX CCD-based diffractometer equipped with a graphite monochromated Cu−Kα radiation source (λ = 1.54178 Å) for rac-[Ir1]Cl, rac-[Ir3]PF 6 , and rac-[Ir4]PF 6 and MoKα (λ = 0.71073 Å) for rac-[Ir5]PF 6 . The highly redundant data sets were integrated using SAINT 6 and corrected for Lorentz and polarization effects. The absorption correction was based on the function fitting to the empirical transmission surface as sampled by multiple equivalent measurements with the program SADABS. 7 The software package WINGX 8,9 was used for space group determination, structure solution, and refinement by full-matrix leastsquares methods based on F 2 . A successful solution by direct methods provided most nonhydrogen atoms from the E-map. The remaining nonhydrogen atoms were located in an alternating series of leastsquares cycles and difference Fourier maps. All nonhydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were placed using a "riding model" and included in the refinement at calculated positions. CCDC reference numbers for rac- Measurements of UV−vis Absorption and Photoluminescence Spectra. UV−vis absorption spectra were recorded in the 200− 1100 nm spectral range by a Shimadzu UV-2450 spectrophotometer, using 10 mm quartz cells, while excitation and emission spectra were recorded on a FLS980 spectrofluorometer (from Edinburgh Instruments) equipped with triple grating turret monochromators and a Red PMT Sphere detector. The F980 spectrometer operating software was used to collect and process fluorescence data. Samples of 1 × 10 −5 M solutions in CH 3 CN were prepared and deoxygenated in a Schlenk using freeze−pump−thaw technique. Then, the solutions were kept under inert atmosphere in quartz cuvettes equipped with Teflon septum screw caps for all the luminescence measurements. All optical measurements were made at room temperature.
The luminescence emission spectra were recorded by exciting at 405 nm with a xenon arc lamp, and the maximum emission wavelength was measured from 420 to 800 nm. The photoluminescence quantum yields (PLQY or Φ) were calculated by detecting all sample emission through the use of an integrating sphere. For the determination of the luminescence lifetime of compounds [Ir1]Cl−[Ir5]Cl, the fluorescence decay was measured on a FLS980 spectrofluorometer equipped with a TSCPC laser and a REDPMT detector. The F980 spectrometer operating software was used to collect and process luminescence lifetime data. The instrumental parameters used were as follows: λ ex = 405 nm, Δλ ex = 0.2 nm, λ em = 648 nm, Δλ em = 4 nm, 2000 channels, integration time = 1 μs, iris setting = 100.
Electrochemical Measurements. Electrochemical measurements were performed using a portable potentiostat/galvanostat PalmSens3 (PalmSens) equipment controlled by the software PSTrace4 Version 4.4.2. All experiments were carried out using a three-electrode cell with a glassy carbon disc (diameter = 3 mm) as the working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl (MF-2052 BASi) reference electrode separated from the bulk solution by a Vycor frit. Oxygen was removed from the solution by bubbling argon for 10 min and keeping the current of argon along the whole experiment. The measurements were recorded for acetonitrile solutions of the complexes (5 × 10 −4 M) in the presence of [ n Bu 4 N][PF 6 ] (0.1 M) as the supporting electrolyte by CV at a scan rate of 100 mV·s −1 in a clockwise direction. Ferrocene was added at the end of all the experiments as the internal reference. The potential experimentally determined for the redox couple Fc + /Fc was E 1/2°= 0.455 ± 0.002 V vs Ag/AgCl. Therefore, the experimental redox potentials were calculated from the corresponding voltammograms as • E°(vs AgCl/Ag) = (E ap + E cp )/2, for reversible peaks where E ap and E cp stand for anodic and cathodic peak potentials, respectively. However, for irreversible peaks, the potentials were calculated as either the E ap maximum or E cp minimum.
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